Frequency modulated radio locator system and indicator



Septv 2; i947. Y'R. c. SANDERS, JR 2,4253@ VFREQUENCY MODULATED RADIO LOCATOR SYSTEM AND INDICATOR www @Y an caw Ww Snventor SepD 2, i947. R. QSANDERS, .l

FREQUENCY MODULATED RADIO LOCATOR SYSTEM AND INDICATOR 5 sheets-snee; 2

Filed July 26, 1945 6 mm ,ww W5 )Werff/c Fonti W11@ mwww@ "y" TME ....www A... n

Sept 2, 1947. f2,426,901

FREQUENCY MQDULATED RADIO LOCATOR SYSTEM ANDINDICATOR R. C. SANDERS, JR

Filed July 2e, 1945 A5 sheetsshet 5 i@ I I lv 56 FLT Fo/Pcf afm? Patented Sept. 2, 1947 UNITED srArss orio Royden C. Sanders, Jr., Hightstown, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application July 26, 1943, Serial No. 496,246

n 3 Claims. 1

My invention relates to object locating and distance measuring systems of the type utilizing the transmission and reflection of radio waves or of other suitable waves. The invention relates particularly to multiple frequency indicators and'to their use inan object locating system of the frequency-modulated type whereby simultaneous indications of a plurality ofl reecting objectsmay be obtained.

Distance measuring systems of the frequencymodulated type are described in Bentley Patent 2,011,392 and in Espenschied Patent 2,045,071. In .these systems, the radiated frequency-modulated wave is reflected from the earths surface or from an aircraft or other object to be located and the reflected wave is received by means including a heterodyne receiver located in the vicinity of the transmitter. The heterodyning or mixing signal for the receiver is obtained directly from the transmitter whereby the receiver output.A includes a signal of beat frequency which frequency is a function of the time required for the radiated signal to reach the refleeting object and return to the receiver.

An object of the present invention is to provide an improved object locating and/or distance measuring system for indicating a plurality of reflecting objects.

Another object of the invention is to provide an improved multiple object or target indicator for object locator systems of the cyclic frequency-modulated type.

A further object of the invention is to provide an improved frequency analyzer.

A still further object of the invention is to provide an improved multiple target indicator or frequency analyzer of the vibrating reed type.

In practicing one preferred embodiment of the invention, the multiple frequency indicator comprises a plurality of indicating reeds all of which are tuned to respond to driving forces of the same frequency or narrow band of frequencies. For example, each reed may be deected or vibrated Iby any applied force, such as a magnetic field or an electric field, that varies in intensity at a rate of from (l to 60 cycles per second. Adjacent to each reed there is located the periphery of a rotatable disc which has a sine wave curve on the periphery for varying the magnetic or electrostatic pull between the disc and the reed. A large number of reeds and discs may be provided, the number depending upon the frequency range to be covered and the accuracy desired. Each disc has a different number of sine waves out on the periphery. For instance,

2 assuming the use of magnetic bias as described hereinafter, if there are 99 discs (and 99 reeds), the first disc may have two sine Waves cut on it, the second disc three sine waves, etc., the last disc having 100 sine waves cut on it. Thus, if rotated at the rate of 50 rotations per second,

Vthe group of discs will produce magnetic force pulsations at the reeds number 1 to 99 having pulsation rates or frequencies from 100v to 5000 per second, respectively.

Assuming that the indicator is employed in a frequency-modulated distance measuring system, the visual frequency indications are obtained by applying the beat-frequency output signal of the detector (after suitable amplification) to the indicator in such a way as to vary the field between the several discs and their associated reeds. This detector output signal may include frequencies from 0-5000 per second, for example. .These signal pulsations beat with the pulsations caused by the sine wave configuration of the discs whereby the field pulsations at one particular reedwill fall within they response range of the reeds, assumed to be from 0 to 60 cycles, and this reed will vibrate or deflect. Since the eld pulsations atv this one reed have been produced by a particular beat-frequency signal in the detector output, the reed vibration indicates the presence of this particular beat-frequency signal and, therefore, the presence of a reflecting object a corresponding distance away.

The invention will be better understood from the following description taken in connection with the accompanying `drawing in which Figure 1 isa diagram of a frequency-modulated radio locator system which includes a multiple target indicator constructed in accordance with one embodiment of the invention,

Figure 2 is a view taken on the line 2 2 of Fig. 1, l

Figure 2a is a plan view of one of the tuned reeds used in the indicator of Fig. 1, v

Figure 3 'is a view taken on the line 3-3 Vof Fig. 1,

Figure 4 is a group of graphs that are referred to in explaining the invention,

Figure 5 is a side view of an electrostatic type indicator embodying the invention, together with a circuit diagram of part of the associated circuit,

Figure 6 is a view taken on the line 6 6 of Fig. 5, and

Figure 'l' is a group of graphs that are referred to in explaining the operation cf the improved indicator,

In the several figures, like parts are indicated by the same reference character.

Fig 1 shows an embodiment of the invention as applied to a frequency-modulated distance measuring system comprising a radio transmitter IB and a frequency-modulator II for cyclically modulating the transmitter carrier Wave in accordance with a modulating signal such as the triangular wave I5 (Fig. 4) that is supplied to the modulator from a pickup device I2. The frequency-modulated wave is radiated from anantenna I3 to the reecting objects and the reflected signal is picked up by a receiver antenna I4 and supplied to a beat-frequency'detector I6. A heterodyning signal is also supplied directly from the transmitter I0 to the detector I6 where it beats with the reiected signal to produce a beat-frequency signal in the output circuit of the detector. Such a beat-frequency signal is illustrated by the graph 20 in Fig. 4. The heterodyning signal and the signal radiated from the transmitter antenna I3 are identical.

The beat-frequency output signal from the detector I6 preferably is amplified by an audio ainplier I1 and supplied to the signal input coil I8 of a multiple-reed frequency indicatorl I9 that is designed in accordance with one embodiment of the invention. v

Referring to Figs. 1 and 2, the indicator I9 corriprises a plurality of discs A, B, C, D, E, etc., which are made of steel or other magnetic material. These discs are mounted on a shaft 2|, also of magnetic material, which is rotated by a motor 22. The speed of rotation may be 50 rotations per second, for example. Each of the discs A, B, C, etc., has a sine wave configuration cut on its periphery. This is illustrated in Fig. 2 where" the disc D is shown. A plurality of tuned reeds A', B', C', etc. are mounted adjacent to the peripheries of the discs A, B, C, etc.,'resp`ectively, each reed being tuned for'maximum response at the same frequency or narrow band of frequencies. For instance they may be tuned Vfor maximum respOsc'at andin the region of 25 cyclesper second, the reeds being 'damped as indicated in Fig. 2a so that the reed response resembles that of a low pass filter. Inthe example assumed, each reed will respond to any frequency in the range of from 0 to 60 cycles. The Vreeds A', B', C', etc. areinounted on asupporting bar 23 whichmay be bent back on itself to provide asection 23 upon which the input coil I8 is wound. vBoth the' indicator reeds and the bar 23 are Vof magnetic material so that by positioning' the end of the section 23' close to the disc A there isprov'ided a closed magnetic circuit for the coil I8.

The indicator vmay be operated either with or without magnetic bias. For example, the DLC. component Yof theranode current in the aud-io amplifier I1 may flow tlughthe l I8 to provide a` magnetic bias; or the connection to the coil I8` may include a capacitor whereby only alternating current flows therethrough;

The operation will first be consideredassuming the use of linagnr'etic bias. -If there is-no beatfrequency signal appearing in the detector output circuit, the only current flowing through the coil I8 Vwill be the D.C. component of the audio amplifier plate current. In that case, if the discs A, B, C, etc., were stationary, there would be a steady magnetic field between the discsand the associated tuned reeds and a steady magnetic pull on the reeds. When the discs are rotated, however, the magnetic force exerted on each reed varies sinusoidally as illustrated in Fig. 4 Where the graph represents the variations introduced by one particular disc. The rate of this varia tion at each reed depends upon the number of sine Waves cut on the associated disc. Since these pulsations or variations in the magnetic iieldoccur at frequencies that are higher than the frequency response range of the tuned reeds, the reeds are not deflected thereby. Such pulsations Will have a range of from 100 to 5000 cycles 'per second, inV the Vexample assumed,

' eld variation represented by the graph 25. Since the beatafrequencysignais from the detector I6 lie within the range of from 0 to 5000 'cycles per second ii the eXample assumed, there Will be beat uctuations in the magnetic force of the required low frequency (0 to 60"c'y`cles)' at lcertain indicatorreeds to cause their Vibration or deflection'. Therefore, the 'deflection of a particular reed indicates the presence of aA signal having a particular beat frequency and thus indC'ateS the presence of reflecting Object a Y(301"- respondiiig distance away.

:In order to avoid any error in the frequency indication due to' YVaii'ttios'inthe speed of the rfot'o'r '22,itis desirable to maintain synchronis'm betweentlie iiiotor' vspeed -ad Ythe frequencyrro'dulatirig signal. This ay be done Acori.- Veietly' loypenarently-reofdig the ihodlating" signal, siichnas atriargiilar waye. on afinagntic tape/25. The` tape 26 Ifriay be carried by a non-magnetic wne'el '21, (rig. s) that is keyed or" otherwise' fastened to the'sh'aftl2 I. Thev modulatiigsi'gnal is taken l the tapeZG by the pickup unit |'2 and supplied to the modulator l I.

With this arrangement, any variation in rrotor speed 'will change the' "frequency modulation rate fm 'and' cause achange the'fredueneyfr of the received sig-nal reflected Vfrom a. target .a given distance away. This is shown bythe equation f apra) (D2-1B) Wher'elj. is the distance to the target in feet and B is :the band width ofthe frequency modulation 'in me'gacycles, "the frequencies fr and fm being in cycles per second. 'rhereforawnile any motor speed'inc're'asegfor example, will increase the rate` of vthe, Vmagnetic eld pulsations caused by" the 'discs'A, B, etc., it will cause a like increase in the frequency of the' field pulsations produced bythesigrial froi'ri a given target and the beat reduen'c'yof these two pulsatirs Will remain unchanged. A l

In practicing the. invention, the indicator may be operated either 'with' a bias ofiinagnetic' flux as assumed in the foregoing description, or w'ithno bias, i. e., :with no DL-C.` component lin the flux. In eitherlcase the beatfrequency discontinuity at the end 'of .a frequency modulation sweep, at times, may be such that the beats produced vduring one sweep oppose-the tuned reed vibrations thatV have been set up by apreceding sweep. This might result in poor indication for a particular target if such discontinuity remained the same for each successive sweep. However, this would be the case only if the reflection were from a strictly stationary target.

The more usual situation will be where the indicator is used to locate moving objects such as surface ships or aircraft. In that case, the beatfrequency discontinuity will be different at the end of each successive frequency-modulation `sweep and no discontinuity of the type causing poor indication will last 10ng enough .to cause failure of tuned reed response for any target.

There will be only one beat cycle or less during each frequency modulation sweep for actuating the reeds in the apparatus described where the reeds respond to frequencies from to 60 cycles per second and Where the frequency modulation is at the rate of 50 per second. If the tuned reeds have maximum response in the region of 25 cycles per second, then the received signal that will give maximum reed response will produce only about a half cycle of magnetic force to actuate the reed, i. e., there will be only this fractional cycle before the next discontinuity occurs in the beat frequency. This is not objectionable because the reeds can be swung to full amplitude of deflection in a half cycle or less.Y

There are certain differences in the design and operation of an indicator having magnetic bias and one having no magnetic bias which are noted below.

With magnetic bias In this case the frequency of the magnetic force (the magnetic force beat) that vibrates a reed is equal to the signal frequency minus the sine wave disc frequency.

As to the effect of a beat frequency discontinuity, any discontinuity tends to impair the tuned reed response somewhat.

Without magnetic bias In this case the frequency of the magnetic force (magnetic force beat) that vibrates a reed is equal to twice the signal frequency minus the sine wave disc frequency. This means that the number of sine waves cut on each disc should be twice the number of sine waves cut thereon for use with magnetic bias. Thus a varying force such as shown by the graph 40 in Fig. 4 beats with the varying force caused by the signal 20, for example, to produce the beat force represented by the graph 45.

The effect of a beat-frequency discontinuity differs from the case where bias is used in that a 180 phase change has no effect. Any other discontinuity, however, impairs the reed response somewhat.

Figs. and 6 show an embodiment of the invention that differs from that of Fig. 1 in that a varying electric force instead of a magnetic force is utilized for driving the indicator reeds A', B', etc. In this design the discs A, B, etc., and the other parts which should be of magnetic material in the design of Fig. l need only be of conducting material so that a suitable voltage difference may exist between a disc and its associated reed. A D.-C. voltage such as 500 volts may be applied to the indicator reeds through the secondary 3S of a transformer 31 while the discs A, B, etc., are held at ground potential by connecting the shaft 2| to ground through abrush 38.

The detector output or beat-frequency signal is supplied to the primary winding 39 of the transformer 31 whereby the beat-frequency signal voltage varies the electric field between the reeds A', B', etc., and the discs A, B, etc. These variations inthe electric field beat with the variations in the eld caused by the rotating discs whereby the tuned reed opposite a particular disc will be vibrated by a particular beat-frequency signal just as described in connection with Fig; l.

In the case where an electric vfield is used, the previous comments regarding the use of bias or no bias apply. In Fig. 5 there'is illustrated the use of an electric field bias which is provided by the +500 volts, the output of `amplifier I1 never being sufficient to reduce the electric field at the reeds to Zero. If the indicator of Fig. 5 is to be operated with no bias, the lower end of coil 36 is grounded and the number of sine waves on the discs are doubled.

The response of the indicator reeds to different beat-frequency signals is illustrated inv Fig. 7 for the particular design constants-that have been assumed and for the case where there is either a magnetic bias or an electric field bias. The graph 48 shows that the reed opposite the 450 cycle disc (9 sine waves on disc) will respond to applied beat-frequency signals over the comparatively narrow frequency range from 390 cycles per second to 510 cycles per second. If the beat frequency of the applied signal is 450 cycles, there will be a zero force beat and the reed will be deflected away from its center Aposition. If the beat frequency of the applied signal is either 425 cycles or 475 cycles, for example, the force beat will be a 25 cycle beat and the reed will vibrate with the same response in both cases.

It will be noted from a comparison of the graphs 48, 49 and 5l that the response ranges of adjacent reeds are made to overlap. Therefore, while the reed opposite the 4:50l cycle disc responds alike to applied beat-frequency signals of frequencies above or below 450 cycles, it is only the frequencies above 450 cycles that will cause the reed opposite the 500 cycle disc to respond also.

It may be preferred to have little, if any, overlap in the response of adjacent reeds. This condition is illustrated by the broken line response curves 48a, 49a and 5Ia, representing a case where the upper response limit of each reed is about 30 cycles per second.

Also, if preferred, the indicator reeds may be tuned comparatively sharply to have a response such as shown by the graphs 48h, 49h and 5H).

It will be noted that the response ranges fork adjacent reeds overlap so that there is no dead spot in the frequency response of the indicator.

I claim as my invention:

l. A radio locator system comprising means for cyclically frequency modulating a radio wave and for transmitting it to a reflecting object, a detector, means for applying to said detector both the reffected wave and the frequency-modulated wave direct from the transmitter to produce a beat-frequency signal, a frequency indicator comprising a plurality of rotatable discs each having its periphery cut in the configuration of a sine wave and each disc having a different number of sine waves cut thereon, a plurality of similarly tuned indicator reeds positioned adjacent to the peripheries of said discs, respectively, means for establishing a force between said discs and said reeds which varies with the change in spacing therebetween as the discs Laureano-1 rare rotated, andmeans for further varying :said force `in accordance Awith :said heat-"frequency :signal for :producing beats fin 'said vLforce whereby 1a vgiven -frequencyicomponent inthe applied sig- :nal :produces beats `differing 2in :frequency at the different reeds, one of .said beats having a `frequency Within the .response range of lsaid .reeds -Whereby it deects the reed that is .located where said one beat occurs, said means for 'frequency .modulating the radio wave .comprising a 4record of the frequency-modulating wave, means for rotating said vrecord synchronously with said discs, .fand a pickup de-vice for taking signal oif Isaid record.

l2. A radio locator system .comprising means for cyclically frequency modulating -a radio wave and 'for transmitting Vit to a reflecting vobject, `a detector, Kmeans for :applying 'to said -detector both -the 'reected vWave and the wave di- -rect from the transmitter to produce la -beatfrequency signal, a frequency indicator cornprising a plurality of similarly 4'tuned indicator elements, vmeans for 'applying to said ltuned elements an electrical force that varies in accordance vwith said 'beat-frequency signal, means rfor varying said electrical `-force at a di'ierent `periodic rate 'at each fof said tuned .elements for producing beats `in said force whereby `a given frequency Acomponent in the applied signal produces beats dilering in Afrequency at the different tuned elements, .one o'f said beats having 'a frequency within the response Yrange -of said tu-n'ed element whereby Ait actuates the tuned Aelement that -is located where said one 'beat ocfcurs, and means for synchronizing the frequency modul-ation -of Asaid radio Wave Wit-h the production of said periodic variations.

f3. .A lradiolocator .system comprising means for kcyclically 'frequency modulating a .radio lwave ian-d 'for transmitting it to a reflecting .obj'e'ctpa detector, .means for applying ito said detector both the reected Wave andthe xwave direct from the vtransmitter to produce :a vbeatdrequencysignal, :a lfrequency indicator compris- 'ing :a :pluralityof :similarly tuned indicator @elesments, vmeans yfor applying to Asaid tuned elements an :electrical force ,that Ivaries vin accordlance .with said 'beat-frequency signal, `means .for varying saidfelect'rical force at `a different periodic rate at each `of saidtuned elements 'for producing beats .in :said force whereby a :given frequency'component in the applied signal produces :beats diiering in frequency Aat the differ- .ent tuned elements, one -of said `beats havingr a :frequency within fthe response `range -of said 'tuned elements Vwhereby it -actuates :the tuned element that is :located Where said 'one beat occurs, said means for frequency modulating lthe lradio Wave comprising -a record-ofthe frequencyvmodulating Wave, and means ,for taking signal off Isaid record synchronously with the ,production Vof -said A,periodic rate variations.

-ROYDEN C. SANDERS, Ja

REFERENCES n("/I'LIIED TIlhe Ifollowing references .are of record lin the -le o'f this patent:

UNITED 'STATES 'PATENTS Number Name Date A' 831,180 McGarvey Sept. 18, 1906 2,=133,`1f50 A'Rich Oct. 11, 1938 1,993,326 Hart 15, 1935 .2,280,109 Varela Apr.. 2l, frgi 

